1. Field of the Invention
The present invention relates to an optical waveguide, more specifically, to an optical waveguide in which dielectric layers surrounding a metallic thin film for propagating light are formed to have a different refractive index, and a propagation loss of the metallic thin film is minimized by a combination of thickness and refractive index of the dielectric layers, thereby implementing long-range light transmission.
2. Description of the Related Art
A surface plasmon is an oscillating wave which propagates along an interface between materials with permittivities having a reverse sign. In general, a surface plasmon exists at the interface between metal having the negative sign and a dielectric having the positive sign, and can be excited by electrons accelerated at high speed and optical waves. An electromagnetic wave which is coupled to a surface plasmon so as to propagate is referred to as a surface plasmon-polarion (hereinafter, referred to as “SPP”).
Since the wave vector of the surface plasmon is larger than those of peripheral materials, the SPP is bound to a metal surface. Therefore, it can be considered that the interface between metal and a dielectric is a two-dimensional optical waveguide having a vertical confinement condition.
In view of the optical waveguide, the SPP to be generated at the interface between metal and a dielectric is effectively bound to a metal surface, while a propagation distance is as short as dozens of mm in a visible-ray region. However, when the thickness of metal is limited to several nm to dozens of nm such that the SPP propagating at the interface is coupled, long-range transmission of light can be realized. This is referred to as a long-range surface plasmon polariton (LR-SPP) mode. The field profile of the LR-SPP mode is widely distributed in a dielectric around a metallic thin film. Therefore, a propagation loss of light is small, and a coupling characteristic with optical fiber is excellent. Accordingly, the LR-SPP mode is applied to various optical element fields.
In general, the SPP mode of an optical waveguide in which a metallic thin film is interposed is divided into the LR-SPP mode and an SR-SPP (short range surface plasmon-polariton) mode. In the LR-SPP mode, the metallic thin film is formed to have a thickness of less than dozens of nm such that light is propagated by a long distance. In the SR-SPP mode, light is propagated through a waveguide having a relatively small size. The LR-SPP is currently applied to an optical waveguide element which is used in optical modulators, switches, couplers, filters, and optical sensors.
Such a conventional optical waveguide to be applied to various fields is disclosed in U.S. Pat. No. 6,442,321. Hereinafter, the construction thereof will be briefly examined, and the problems thereof will be described.
FIG. 1 is a sectional view of a conventional LR-SPP mode optical waveguide. The optical waveguide 1 includes a metallic thin film 2 formed of a material having high charge density or a negative permittivity and a dielectric layer 3 surrounding the metallic thin film 2, the dielectric layer 3 having a width w and thickness t in the same dimension as light emitted from the metallic thin film 2.
The metallic thin film 2 may be formed with a line, a curved line, a curved surface, or an inclined surface, depending on the structure of the optical waveguide.
In the conventional optical waveguide, when light is incident from the outside through the metallic thin film 2, the light corresponding to the LR-SPP mode is propagated along the metallic thin film 2. At this time, while the light is propagated along the metallic thin film 2, a propagation loss inevitably occurs. In order to reduce such a propagation loss, an amount of light to be propagated inside the metallic thin film 2 should be reduced. Therefore, the thickness t or width w of the metallic thin film 2 should be reduced.
However, since there a limit in manufacturing the metallic thin film 2, there is a limit in reducing the thickness t and width w of the metallic thin film 2. Therefore, there are difficulties in reducing a propagation loss of the metallic thin film 2.
In order to minimize a propagation loss through the metallic thin film 2, the metallic thin film 2 should be formed to have a thickness of less than 0 nm and a width of less than 1 μm. However, there is a limit in manufacturing a metallic thin film having a thickness of less than 10 nm and a width of less than 1 μm by using a current technique.
For wavelengths of 1550 nm and 633 nm which are representative optical-communication wavelengths to be used in the conventional optical waveguide, an effective refractive index, a propagation loss, and a loss of light (coupling loss) caused by the coupling to optical fiber are calculated as follows.
Since all the optical elements of the optical waveguide are connected to optical fiber, the coupling loss is considered. The metallic thin film 2 formed of gold has a permittivity ε1 of −131+i×12.65 and −19+i×0.53 corresponding to the wavelengths of 1550 nm and 633 nm, respectively. The dielectric layer 3 surrounding metallic thin film 2 has a permittivity ε2 of 2.25.
The thickness t and width w of the metallic thin film are set to 5 μm and 20 nm for the wavelength of 1550 nm and to 2 μm and 20 nm for the wavelength of 633 nm, respectively. The diameter of the optical fiber is set to 10.5 μm for the wavelength of 1550 nm and to 4.3 μm for the wavelength of 633 nm.
The effective refractive index, the propagation loss, and the coupling loss, which are calculated under the presented condition, are 1.50069, 7.44 dB/cm, and 0.24 dB for the wavelength of 1550 nm and 1.51393, 13.45 dB/cm, and 3.37 dB for the wavelength of 633 nm, respectively.
A propagation distance up to a spot, where the intensity of light decreases to 1/10 in consideration of the propagation loss, is about 1.34 cm for the wavelength of 1550 nm and mo more than 0.74 mm for the wavelength of 633 nm. Therefore, when the LR-SPP mode optical element is used, the element should be manufactured to have a length of 1 cm for the wavelength of 1550 nm and a length of less than 1 mm for the wavelength of 633 nm, respectively.